1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet into an image forming apparatus.
2. Discussion of the Background
In recent years, image forming apparatuses have generally been provided with a sheet feeding apparatus which individually feeds a sheet to be copied (hereinafter called a sheet) or the like, which is contained in a sheet feeding cassette. Various kinds of sheet feeding devices can be utilized. For example, a sheet feeding device having a manual sheet feeding entrance can be provided in addition to a sheet containing section such as a sheet feeding cassette or the like for selective use as a starting position of a sheet feeding operation.
In such a sheet feeding apparatus, a sheet containing section and the manual sheet feeding entrance are communicated with one sheet conveying path, and one of the sheet containing section and the manual sheet feeding entrance is selected as a starting position of the sheet feeding operation. There are typically two devices provided at the sheet conveying path. One device is a registration mechanism which temporarily stops the sheet which is individually being fed from the sheet containing section or the manual sheet feeding entrance. A second device is a registration sensor which detects the position of the sheet as it is fed from the sheet containing section or the manual sheet feeding entrance to the registration mechanism.
In such a sheet feeding apparatus, for example, in a case of individually feeding the sheet from the sheet containing section to an image forming section, the sheet is temporarily stopped by the registration mechanism to adjust the timing for when the sheet is to be conveyed into the image forming section. Presently, this is performed by detecting a position of the sheet with the registration sensor as the sheet is fed towards the registration mechanism. The sheet is held by the registration mechanism, temporarily stopping motion of the sheet until after a predetermined time period has passed from the detected time point. The sheet is then fed towards the image forming section at an appropriate timing.
In a case of feeding the sheet from the manual sheet feeding entrance, when an operator inserts a sheet to the manual sheet feeding entrance, the sheet is started to be fed after being detected by a special sensor. In a manner as stated above, the sheet is also detected by the registration sensor and temporarily stopped by the registration mechanism. The sheet is then fed towards the image forming section. The sheet feeding timing is thus adjusted by the temporary stop.
Further, since the registration sensor detects the position of the sheet as it is fed towards the registration mechanism in the aforementioned manner, an occurrence of a sheet jam can be judged by a jam detecting circuit if the registration sensor detects the position of the sheet at an undesirable time. For example, when the registration sensor continuously detects the position of the sheet after a predetermined time period has passed from a time in which the registration mechanism has ended the temporary stopping of the sheet, it is assumed that a sheet jam has occurred at a position of the registration sensor, and accordingly, the jam detecting circuit executes lighting of a jam lamp or sounding of a buzzer for reporting the occurrence of the sheet jam to the user.
In the aforementioned sheet feeding apparatus, a sheet can be fed from either one of the sheet containing section and the manual sheet feeding entrance. However, as mentioned above, a special detecting sensor is required for detecting the position of the sheet inserted into the manual sheet feeding entrance. There is also presently in use a sheet feeding apparatus having a registration sensor located at a position where the sheet is inserted into the manual sheet feeding entrance so that the manually inserted sheet can be detected without the need for a special sheet detecting sensor.
This type of sheet feeding apparatus requires no special sheet position detecting sensor at the position of the manual sheet feeding entrance. This is because the position of the sheet inserted into the manual sheet feeding entrance is detected by the registration sensor.
However, in this kind of sheet feeding apparatus, the jam detecting circuit judges that a sheet jam has occurred when a power switch is turned on in a state when a sheet remains in the manual sheet feeding entrance, because the position of the sheet is detected at an undesirable time by the registration sensor. Therefore, the operator is required to perform unnecessary operations for removing a jammed sheet, in spite of the fact that no actual sheet jam has occurred.
The case where the sheet feeding apparatus is powered up in a state in which an inserted sheet is present in the manual sheet feeding entrance can easily and frequently occur. For example, when a power switch of the image forming apparatus is turned off when a sheet has been inserted into the manual sheet feeding entrance, and later, the power switch is turned, while the sheet remains in the manual sheet feeding entrance, a false sheet jam judgment will be made. This situation can occur in daily use of the image forming apparatus and can cause the above-mentioned problems. In other words, the operator may typically turn off the power switch of the image forming apparatus during the night time or the like when the image forming apparatus is not being used, without removing the manually fed sheet from the manual sheet feeding entrance. In addition, for safety reasons the image forming apparatus is typically set so that the power is automatically turned off when the operator opens the main body or a sheet conveying path, for example, for removing a jammed sheet. Therefore, after removing a jammed sheet, even though the power is automatically turned on by returning the main body or the sheet conveying path to the closed state, if the sheet inserted into the manual sheet feeding entrance remains, there occurs a situation in which the sheet feeding apparatus is started while the sheet inserted into the manual sheet feeding entrance remains, thus causing the above-noted erroneous jam detection problems.